1. Field of the Invention
The present invention relates to a method and a device for sealing the place of penetration of a pipeline, especially a riser pipeline, in the wall of a submerged structure, which pipeline extends generally horizontally at the place of penetration and is connected to a generally vertical section inside the structure, wherein an annular sealing device which is flexible at least in the axial direction of the pipeline is placed coaxially with the pipeline at the place of penetration, one end of the sealing device being connected tightly to the wall while its other end is tightly connected to the pipeline.
2. The Prior Art
One of the most desirable methods for transporting fluids like oil, gas, slurry, and water to and from offshore structures resting on the sea bed is through pipeline systems. The part of the pipeline system resting on the sea bed is called a submarine pipeline, while the part which is connected to the structure is called the riser.
The riser is vulnerable to many problems which range from corrosion and overstressing to damage caused by supply vessels. Because of these problems the riser is subjected to thorough design considerations. In order to avoid wave loads and damage due to supply vessels, there has been a tendency in resent years to place the riser inside the platform structure. This necessitates the penetration of the pipeline through the wall of the platform structure. The area inside the platform structure where the riser is located can either be a wet or dry environment. Since the riser is located inside the platform structure and thus is in the vicinity of areas where people work, any damage to the riser can cause loss of human life. In order to maximize safety, the riser inside the platform structure must be inspected periodically and minor damage repaired to avoid catastrophic failure.
Underwater inspection and repair of the riser in a confined space inside the platform structure are very hazardous operations. This space must therefore be capable of being dewatered in order to allow inspection and/or repair of the riser in a dry environment.
Therefore, whether the riser is in a dry or wet area, a safe sealing system must be provided at the place of penetration of the pipeline in the wall of the pipeline structure in order to ensure maximum safety during repair, maintenance and inspection.
There are situations where large relative movement takes place between the structure and the submarine pipeline. Such movements may be caused, for instance, by expansion of the pipeline due to temperature and/or pressure in the pipeline and movement of the structure due to external influences like waves, current, wind and soil deformation.
Large relative movements between the pipeline and the platform structure may thus give rise to high stresses and loads in the pipeline and in equipment and parts of the structure directly in the way of such movements, such as the seal and pipe support systems.
From U.S. Pat. No. 4,009,584 it is known to seal the place of penetration of a pipeline in the wall of a submerged structure, wherein the pipeline by casting is fixed to the wall of the structure at the place of penetration by means of epoxy or the like. The epoxy is said to be sufficiently flexible to absorb minor relative motion between the pipeline and the structure, that is, minor movements or in the order of magnitude of a few centimeters. One can assume, however, that there may occur forces between the pipeline and the structure which are high enough to destroy the epoxy joint. In order to safeguard against catastrophic results of such an occurrence a safety seal can be arranged around the joint, which seal generally has the form of a stuffing box which is meant to prevent the major leakage until the main seal can be repaired. In one of the embodiments shown the safety seal is connected to a bellow-like sleeve in order to give the safety seal a certain ability to move together with the pipeline without the occurrence of sliding motion--and thereby leakage--between these elements. However, the safety seal remains only a temporary emergency solution until the main seal can be repaired. Such a pipeline being rigidly held in or near the wall of the structure will readily be subjected to high loads at this place since it will serve as a fixed point when the pipeline seeks to move as a result of changes in temperature and/or pressure when the pipeline is put into service. These loads give rise to stresses which may be so high that danger of rupture results. Furthermore, such stresses can often lead to accelerated corrosion and cracking.
The object of the present invention is to provide a method and an apparatus of the type mentioned whereby the above-mentioned drawbacks and deficiencies are generally avoided.